Monomeric, but polymerizable, lecithins with diacetylenic fatty acyl chains, such as l,2-bis(10,12-tricosadiynoyl)-.w-glycero-3-phosphocholine (DCg 9PC), are known to form tubular microstructures when liposomes of these lipids are cooled through their chain melting transition. These lipids are soluble in alcohols and other organic solvents, but when such solutions are diluted with water at appropriate temperatures, precipitates form. From optical and electron microscopy the precipitates are seen to consist of tubules and long, open helical structures with diameters similar to those of the tubules. These helices are all right handed when made from lipid with the naturally occurring chiral head group. For an ethanol/water system the proportion of helices and tubules depends on the ratio of the solvent to nonsolvent, as does the overall length of the tubules. The temperature, lipid concentration, and specific solvent used also affect the nature of the precipitate. For DCg 9PC the tubular and helical microstructures vary from 0.3 to 3 pm in diameter, and from 5 to over 1000 pm in length. The width and pitch of the helical ribbons are variable, resulting in a range of structures from open helices to continuous tubules depending on the solvent system used. Upon exposure to energetic radiation such as UV rays or 7-rays, the diacetylenic units polymerize without causing loss of the helical or tubular microstructure, thereby stabilizing the microstructures. Demonstration of this formation route for tubules suggests that they are thermodynamically stable, not accidental products of deforming liposomes. The existence of this polymerizable helical microstructure that may be an intermediate in the formation of tubules supports previous indications of an underlying helical structure to tubules. This precipitation method also affords a simple method of controlling the dimensions of tubules and a screening method for the discovery of other self-organizing lipid microstructures.

No abstract

No abstract

ABSTRACT:The effects of chemical crosslinking on the thermal and dynamic mechanical properties of a polyurethane system were examined. The polyurethanes were prepared from poly(propylene glycol), a diol; trimethylolpropane propoxylate, a triol; and poly(propylene glycol), tolylene 2,4-diisocyanate terminated, a diisocyanate monomer. The crosslink density was controlled by varying the triol concentration from 10 to 70 mol % and the isocyanate-to-hydroxyl (NCO/OH) ratio from 1.0 to 1.3. All the samples had one glass-transition temperature and no crystalline regions. In addition, there were larger increases in glass-transition temperature over the range of triol concentrations studied than over the range of NCO/OH ratios studied. For all samples, the Dibenedetto equation relating glass-transition temperature to extent of crosslinking fit the data very well. Also, samples with higher crosslink densities had much larger elastic moduli for temperatures above the glass-transition temperature. By assuming the system was a phantom network, approximate crosslink densities for stoichiometric samples were obtained from the dynamic mechanical data and these agreed fairly well with theoretical predictions.